Illuminators for microlithographic projection imagers are generally designed to provide illumination having uniform spatial intensity so that all areas of the field are equally illuminated, especially at a wafer plane where the illumination is incident on a wafer. The uniformity of the spatial intensity of the illumination used has been achieved by painstaking optical design of a number of components guiding illumination from a source to an objective imaging system. Once the illuminator portion of the imager has been designed to assure uniformity of the illumination, then this aspect of the illumination has not been adjustable during the operating life of the equipment.
Experience with such illuminators has revealed several factors that affect or vary the uniformity of the spatial intensity of the illumination, and these factors cause illumination departures from uniformity that can be sufficient to diminish the quality of the microlithographic products made by projection imagers. This led to a recognition of the desirability of improving on the uniformity of the illumination reaching the wafer plane; and in response to this need, I have devised a simple and effective way of varying the spatial intensity of the illumination provided by an illumination system. Making the spatial intensity of the illumination variable in uniformity then allows the illumination system to compensate for factors causing departure from uniformity so that in spite of these factors, illumination can be made uniform in spatial intensity at the wafer plane. In other words, an illumination system allowing uniformity to be varied can change the spatial intensity of the illumination being delivered so that when the illumination reaches the wafer plane, it is uniform in spatial intensity as desired.